1. Field of the Invention
This invention relates to a hetero-semiconductor structure possessing an SOI structure containing a silicon-germanium mixed crystal and a method for the production thereof at a low cost with high productivity.
2. Background Art
In recent years, a method for effecting high speed operation of MOSFET (Metal Oxide Semiconductor Field Effect Transistor) integrated circuits by utilizing a technique called “strained silicon” has been attracting attention. The strained silicon technique consists in enhancing the mobility of electrons or positive holes as the carrier in the channel of the MOSFET by utilizing a silicon layer deformed so as to increase the lattice constant.
To increase the lattice constant of the silicon layer used for the channel, numerous methods for disposing a silicon-germanium mixed crystal adjacent to the pertinent silicon layer have been proposed, as disclosed in JP-A 6-252046, for example. These conventional methods are characterized by depositing a silicon-germanium film by vapor phase deposition on a silicon wafer and thereafter depositing a silicon layer, again by vapor phase deposition. Since the lattice constant of germanium is 4% larger than that of silicon, by controlling the compositional ratio of silicon-germanium mixed crystal, it is possible to impart the necessary strain to the channel layer. More often than not, the proportion of germanium is selected to be in a range of 10-30 mol %.
This strained silicon technique may be used in combination with the so-called SOI (Silicon On Insulator) structure, this combination then known as SGOI (Silicon-Germanium On Insulator). The latter combination can be produced by bonding a first substrate prepared by depositing a multilayer film containing a silicon-germanium mixed layer by vapor phase deposition, and a second substrate furnished with an oxide film, and then removing the first substrate to a certain depth by polishing or etching, as disclosed in JP-A 10-308503. This technique is thus a combination of the SOI technique and the strained silicon technique. Combination with a SIMOX (Separated by IMplanted OXygen) technique (JP-A 4-264724), another typical method for the production of SOI wafers, has also been proposed. For example, a method for forming a buried oxide film in a silicon-germanium mixed crystal layer by depositing a silicon-germanium mixed crystal on a silicon substrate and thereafter implanting oxygen ions and subsequently subjecting the resultant composite to a high temperature heat treatment is proposed in JP-A 9-321307. JP-2001-148473 discloses a method for producing an SGOI wafer possessing a high germanium concentration by the so-called ITOX technique, i.e. by decreasing the thickness of the SOI film by oxidizing the film at high temperature, thereby increasing the germanium concentration in the SOI film.
U.S. Pat. No. 4,975,387 discloses a method for forming a silicon-germanium surface layer by depositing an amorphous silicon-germanium layer on a silicon substrate and oxidizing the resultant composite in an atmosphere of steam.
Production of silicon-germanium mixed crystals by the zone method from a silicon raw material doped with germanium has also been proposed. JP-A 8-143389, for example, discloses a method for forming a bulk single crystal by adjusting the germanium concentration in a liquid phase, thereby controlling the concentration of germanium in the solid phase. These conventional methods of production, however, have entailed numerous problems.
Specifically, when a technique such as that disclosed in JP-A 6-252046 is employed, the silicon-germanium mixed crystal layer intended to impart strain must be sufficiently relaxed until the lattice constant assumes a magnitude conforming to the inherent composition. The relaxation of lattice must be relied on for generation of dislocations. When the dislocation thus generated extends to the region used by the relevant device, it may induce the device to malfunction. Various measures have been proposed to safeguard against this danger. One of the methods, as disclosed in JP-A 6-252046 and JP-A 5-129201, comprises depositing a so-called graded buffer layer, i.e. a layer wherein the compositional ratio of germanium gradually increases during the formation of a silicon-germanium mixed crystal layer by vapor phase deposition, thereby preventing the dislocation from threading to the surface layer. Attaining the necessary compositional ratio of germanium by this technique necessarily results in deposition of thick films, markedly impairing productivity, and heightening the cost of production as a result. U.S. Pat. No. 6,039,803 discloses inclining the main orientation of the silicon substrate by 1-8 degrees from the normally adopted direction of <100>. However, even the use of this method cannot be expected to attain sufficient inhibition of dislocation, since this method entails the problem of requiring deposition of a graded buffer layer.
The combination of the SOI structure and the strained silicon technique which is disclosed in JP-A 9-321307 and JP-2001-148473 does not require formation of a thick silicon-germanium mixed crystal layer as described above. However, the process still requires deposition of a silicon-germanium mixed crystal layer, necessitating a complicated process of production, and heightening the cost of production.
A method for forming an epitaxial layer by depositing an amorphous silicon-germanium layer and subsequently oxidizing the deposited layer in an atmosphere of steam as disclosed in U.S. Pat. No. 4,975,387 requires a separate apparatus for the growth of the amorphous film. Most amorphous film forming devices are susceptible to contamination with impurities. This method, therefore, is not a satisfactory process for the production of wafers for use in high-speed devices desired for present and future production.
Growth of silicon-germanium mixed crystals by the Czochralski technique or by the zone melting technique disclosed in JP-A 8-143389 necessitates a large amount of a germanium raw material. Since the germanium raw material is expensive, the production of a crystal having such a high germanium concentration in the range of 10-30 mol % required for a strained silicon substrate has only little merit commercially. Further, an attempt to grow from a liquid phase a single crystal containing germanium at such a high concentration is technically difficult because growth tends to produce dislocations.